Atomic Claim Pattern

The atomic claim pattern is Monque’s core mechanism for ensuring a pending job is claimed by only one scheduler instance at a time.

This prevents concurrent duplicates, but it does not guarantee that a job can never run more than once. Jobs may be processed again after a crash, a retry, or stale recovery, so workers should be idempotent.

The Problem

In distributed systems, multiple workers might try to pick up the same job simultaneously. Without proper coordination, this leads to:

• Duplicate processing: Same job runs multiple times
• Race conditions: Workers step on each other’s work
• Lost updates: Results get overwritten
• Wasted resources: Redundant computation

The Solution: Atomic Claims

Monque uses MongoDB’s findOneAndUpdate with atomic guarantees to ensure only one scheduler claims each pending job.

How It Works

// Simplified internal implementation
const job = await collection.findOneAndUpdate(
  {
    // Only match jobs that are:
    name: workerName,            // For this worker type
    status: 'pending',           // Not already claimed
    nextRunAt: { $lte: now },    // Ready to run
    $or: [                       // Not owned by another instance
      { claimedBy: null },
      { claimedBy: { $exists: false } }
    ]
  },
  {
    $set: {
      status: 'processing',
      claimedBy: schedulerInstanceId,  // Mark ownership
      lockedAt: new Date(),
      lastHeartbeat: new Date(),
      heartbeatInterval: 30000,
      updatedAt: new Date()
    }
  },
  {
    returnDocument: 'after'      // Return the claimed job
  }
);

Why This Works

1. Atomic operation: MongoDB guarantees the query and update execute as one unit
2. First-writer wins: Only one instance can match and update the same document
3. Immediate visibility: Other instances see the updated document instantly
4. No external locks: No need for Redis, ZooKeeper, or distributed lock managers

Scheduler Instance ID

Each Monque instance has a unique identifier:

const monque = new Monque(db, {
  schedulerInstanceId: 'worker-1'  // Optional: provide your own
});

// Or let Monque generate a UUID
const monque = new Monque(db); // Uses randomUUID()

Tip

Providing a meaningful schedulerInstanceId helps with debugging. Use hostnames, container IDs, or pod names in production.

Viewing Instance ID

// Instance ID is visible in job documents
const job = await monque.enqueue('task', { foo: 'bar' });
// After being claimed:
// job.claimedBy === 'worker-1' (or the UUID)

Multi-Instance Deployment

Scaling Horizontally

Run multiple scheduler instances for high availability:

// Instance 1 (server-a)
const monque1 = new Monque(db, {
  schedulerInstanceId: 'server-a'
});
monque1.register('send-email', emailHandler);
monque1.start();

// Instance 2 (server-b)  
const monque2 = new Monque(db, {
  schedulerInstanceId: 'server-b'
});
monque2.register('send-email', emailHandler);
monque2.start();

// Both compete fairly for jobs
// Each pending job is claimed by exactly one instance at a time

Load Distribution

Jobs are distributed naturally based on claim timing:

Job Queue: [A, B, C, D, E, F, G, H]

Instance 1 claims: A, C, E, G
Instance 2 claims: B, D, F, H
(Actual distribution varies based on timing)

Indexes for Performance

Monque creates the required MongoDB indexes during initialize() to keep claim and polling queries fast.

For the full list of indexes, see Jobs.

// Compound index for claim queries
{ status: 1, nextRunAt: 1, claimedBy: 1 }

// Index for finding jobs by owner
{ claimedBy: 1, status: 1 }

These indexes ensure claim operations remain fast even with large queues.

Claim Lifecycle

sequenceDiagram
    participant W1 as Worker 1
    participant DB as MongoDB
    participant W2 as Worker 2
    
    Note over DB: Job in pending state
    
    W1->>DB: findOneAndUpdate (claim attempt)
    W2->>DB: findOneAndUpdate (claim attempt)
    
    DB-->>W1: Job document (claimed!)
    DB-->>W2: null (already claimed)
    
    Note over W1: Processes job
    
    W1->>DB: Update status to completed
    Note over DB: Job complete, claimedBy cleared

Failure Handling

Worker Crashes

If a worker crashes while processing:

1. Job remains in processing status with claimedBy set
2. lastHeartbeat stops updating
3. After lockTimeout, the job can be recovered on startup (see Heartbeat)

Graceful Shutdown

When stop() is called:

await monque.stop();
// 1. Stops accepting new jobs
// 2. Waits for in-progress jobs to complete
// 3. If shutdown times out, in-progress jobs may remain in `processing`
//    and will be recovered later by stale recovery on startup

Best Practices

1. Use Meaningful Instance IDs

// ✅ Good - Identifiable in logs and debugging
const monque = new Monque(db, {
  schedulerInstanceId: `${hostname}-${process.pid}`
});

// ❌ Less useful - Random UUID (default)
const monque = new Monque(db);

2. Monitor Claim Metrics

let claimCount = 0;

monque.on('job:start', () => {
  claimCount++;
  metrics.gauge('monque.active_claims', claimCount);
});

monque.on('job:complete', () => {
  claimCount--;
  metrics.gauge('monque.active_claims', claimCount);
});

3. Handle Claim Failures Gracefully

monque.on('job:error', ({ error }) => {
  if (error.message.includes('claim')) {
    // Claim contention - normal in multi-instance
    metrics.increment('monque.claim_contention');
  }
});

Comparison with Other Patterns

Pattern	Pros	Cons
Atomic Claim (Monque)	No external dependencies, strong consistency	Requires MongoDB
Redis Locks	Fast, widely used	Additional infrastructure, lock expiry issues
Pessimistic Locking	Simple concept	Blocks other workers, deadlock risk
Optimistic Locking	No blocking	Retry storms under contention

Next Steps

• Change Streams - Real-time job notifications
• Heartbeat Mechanism - Detect stale claims
• Workers - Configure worker concurrency